FIG. 1 depicts a prior art charge integration pixel 10 located at the i-th row and a j-th column of a vision sensor (not shown). Pixel 10 comprises a photodetector represented by current source Iph(i,j), a switch 12, a capacitor 14 and a readout circuit 16. Switch 12 may be any type of sample and hold a device such as a semiconductor. Capacitor 14 functions as a storage device or, specifically, as an integration capacitor.
Pixel 10 generates photocurrent Iph(i,j), which is generally proportional to the intensity of the light impinging on the photodetector. When closed, switch 12 conducts photocurrent Iph(i,j) and charges the capacitor 14. In some embodiments, prior to charge integration, capacitor 14 is completely discharged. In this instance, initial voltage Vinitial across the capacitor 14 is 0 and rises linearly with the accumulation of charge.
If photocurrent Iph(i,j) is time invariant during charge integration, and if switch 12 is shut for an exposure time t=tE(i,j), then an accumulated charge Qa may be calculated as per equation (1).Qa(i,j)=Iph(i,j)·tE(i,j)  (1)
The accumulated voltage Vc(i,j) across capacitor 14 is:
                                          V            c                    ⁡                      (                          i              ,              j                        )                          =                                                            I                                  p                  ⁢                                                                          ⁢                  h                                            ⁡                              (                                  i                  ,                  j                                )                                      ·                                          t                E                            ⁡                              (                                  i                  ,                  j                                )                                                          C            I                                              (        2        )            
where C1 is the capacitance of capacitor 14.
Proof for the following is described in full in the attached Appendix. Herein are some of the equations necessary for understanding the present invention. For ease of understanding, the equation numbers in the specification correspond to those in the Appendix.
The ratio between the saturation voltage
  V  Sat  cand the cutoff voltage
  V  CO  cis defined in equation (5) as the electrical signal dynamic range DRS.
                              DR          s                =                              V            Sat            c                                V            CO            c                                              (        5        )            
From the Appendix it can be seen that for conventional sensors with global exposure time setting, the captured image dynamic range DRL can be defined as in equation (12),
                              DR          L                =                              V            Sat            c                                V            CO            c                                              (        12        )            
For prior art image sensors with globally set exposure time, the captured image dynamic range DRL is equal to the electric signal dynamic range DRS and is exposure time invariant.
Some image sensors have individually per-pixel-controlled electronic shutters, such as those described in U.S. patent applications Ser. No. 09/426,452 “Image Sensor's Unit Cell with Individually Controllable Electronic Shutter Circuit” and Ser. No. 09/516,168 “Image Sensor Architecture for Per-Pixel Charge Integration Control”. For those image sensors, the captured image dynamic range can be shown by equation (17):DRL=DRS·DRT  (17)
where the shutter or exposure time dynamic range DRT is the ratio of the maximum to the minimum exposure time TE.
                              DR          T                =                              t            E            max                                t            E            min                                              (        18        )            
One result from (12) and (17) is that for image sensors with per-pixel-controlled electronic shutters, the captured scene dynamic range DRL may be at most an electrical signal dynamic range DRS times better than the prior art image sensor's dynamic range. For instance, if the electric signal dynamic range DRS is 1,000:1, and the exposure time setup is in the same, then the captured scene dynamic range DRL can be 1,000,000:1, or about 120 db. Thus there is still an incentive to improve the electrical signal dynamic range, since it directly affects the results for image sensors with per-pixel-controlled electronic shutters.
In image sensors with per-pixel exposure time control, since the dynamic range is time dependant and can be significantly improved by efficient management of the exposure time tE, there is a need to develop a method and apparatus for definition of a generally optimal exposure time per cell, or pixel.